Labor share actually went up for 1950 to 1970, and I don’t think increases in automation were absent in that period: http://www.macleans.ca/wp-content/uploads/2012/09/fredgraph_...
It's why in 2018, you can be living paycheck-to-paycheck on minimal wage, but own a flat-screen color TV. Is this a better life then doing the same job for more pay in 1950, where there were no flat-screen color TVs, home appliances were much more expensive, and much less useful? In many respects, yes.
The question is, what will your life be like in 2050? If current trends continue, we expect your share of the productive output of society to go down. Will this be offset by productivity gains (More, cheaper stuff)? Or will limits to growth (Cost of energy, cost of pollution) actually make you worse off?
However, the argument for a current/recent causative link between the two is not a "preconceived notion" but rather a reasoned hypothesis arrived at based on observation of trends and application of logic to economic theories.
It is more reasonable to suppose that:
overall, automation has had a role reducing the labor share (which has been going down despite a couple of brief spikes in the 50s and 60s)
than to suppose:
overall, automation has been increasing the labor share, but this is overshadowed by other causes which have been decreasing labor share.
Thus, if you want to argue for the later supposition, the burden of proof is more heavily on your shoulders to explain the other causes which somehow outweigh the effects of automation on the economy.